MCAT Biochemistry Question 118: Answer and Explanation
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Question: 118
13. For most cells, the extracellular calcium concentration is around 10,000 times higher than the intracellular calcium concentration. What is the membrane potential established by this electrochemical gradient?
- A. –123 mV
- B. –61.5 mV
- C. +61.5 mV
- D. +123 mV
Correct Answer: D
Explanation:
The Nernst equation relates the intra- and extracellular concentrations of an ion to the potential created by that gradient. At physiological temperature, it can be simplified to 
. For calcium, z = +2 (Ca2+) and the ratio of [ionoutside] to [ioninside] = 104. Plugging in, we get.
